Diffusion Tensor Imaging in Alzheimer's Studies.

In this chapter, we describe the use of quantitative metrics of white matter obtained from the diffusion tensor model based on diffusion-weighted imaging in Alzheimer's disease (AD). Our description synthesizes insights not only from patient populations with AD dementia but also from participants at risk for AD dementia (e.g., amnestic mild cognitive impairment, subjective cognitive decline, or familial AD mutation carriers). A reference to studies examining correlations with behavioral variables is also included. Our main message is to caution against the overinterpretation of diffusion metrics and to favor analyses that focus on regions of interest or major white matter tracts for biomarker studies in AD.

Resting-state BOLD functional connectivity depends on the heterogeneity of capillary transit times in the human brain A combined lesion and simulation study about the influence of blood flow response timing.

Functional connectivity (FC) derived from blood oxygenation level dependent (BOLD) functional magnetic resonance imaging at rest (rs-fMRI), is commonly interpreted as indicator of neuronal connectivity. In a number of brain disorders, however, metabolic, vascular, and hemodynamic impairments can be expected to alter BOLD-FC independently from neuronal activity. By means of a neurovascular coupling (NVC) model of BOLD-FC, we recently demonstrated that aberrant timing of cerebral blood flow (CBF) responses may influence BOLD-FC. In the current work, we support and extend this finding by empirically linking BOLD-FC with capillary transit time heterogeneity (CTH), which we consider as an indicator of delayed and broadened CBF responses. We assessed 28 asymptomatic patients with unilateral high-grade internal carotid artery stenosis (ICAS) as a hemodynamic lesion model with largely preserved neurocognitive functioning and 27 age-matched healthy controls. For each participant, we obtained rs-fMRI, arterial spin labeling, and dynamic susceptibility contrast MRI to study the dependence of left-right homotopic BOLD-FC on local perfusion parameters. Additionally, we investigated the dependency of BOLD-FC on CBF response timing by detailed simulations. Homotopic BOLD-FC was negatively associated with increasing CTH differences between homotopic brain areas. This relation was more pronounced in asymptomatic ICAS patients even after controlling for baseline CBF and relative cerebral blood volume influences. These findings match simulation results that predict an influence of delayed and broadened CBF responses on BOLD-FC. Results demonstrate that increasing CTH differences between homotopic brain areas lead to BOLD-FC reductions. Simulations suggest that CTH increases correspond to broadened and delayed CBF responses to fluctuations in ongoing neuronal activity.

Modeling the impact of neurovascular coupling impairments on BOLD-based functional connectivity at rest.

Functional magnetic resonance imaging (fMRI) of blood oxygenation level dependent (BOLD) signals during the resting-state is widely used to study functional connectivity (FC) of slowly fluctuating ongoing brain activity (BOLD-FC) in humans with and without brain diseases. While physiological impairments, e.g. aberrant perfusion or vascular reactivity, are common in neurological and psychiatric disorders, their impact on BOLD-FC is widely unknown and ignored. The aim of our simulation study, therefore, was to investigate the influence of impaired neurovascular coupling on resting-state BOLD-FC. Simulated BOLD signals comprising intra- and extravascular contributions were derived from an adjusted balloon model, which allows for independent definitions of cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) responses, being elicited by a synthetic oscillatory input signal with low frequency (0.05 â€‹Hz) amplitude modulations. BOLD-FC was then defined by correlations between physiological reference BOLD time curves (seeds of seed-based BOLD-FC) and the test BOLD time curves (targets of BOLD-FC) featuring altered physiological variables (CMRO2, CBF, cerebral blood volume (CBV)). Impact of impaired neurovascular coupling on BOLD-FC was investigated for three different scenarios with independent changes in (1) CBF and CMRO2amplitudes, (2) CBF and CMRO2delays, and (3) coupling between CBF and CBV. For scenario 1, we found 'linear' influences of CMRO2 and CBF amplitudes on BOLD-FC: for a given CMRO2 amplitude, BOLD-FC changes from negative to positive FC with increasing CBF amplitude, and increasing CMRO2 amplitude simply shifts this dependence linearly. For scenario 2, CMRO2 and CBF delays had a complex 'non-linear' effect on BOLD-FC: for small CMRO2 delays, we found that BOLD-FC changes from positive to negative BOLD-FC with increasing CBF delays, but for large CMRO2 delays positive BOLD-FC simply diminishes with increasing CBF delay. For scenario 3, changes in CBF-CBV coupling have almost no effect on BOLD-FC. All these changes were not critically influenced by both signal-to-noise-ratio and temporal resolution modulations. Our results demonstrate the importance of alterations in neurovascular coupling for aberrant resting-state BOLD-FC. Based on our data, we suggest to complement BOLD-FC studies, at least of at-risk patient populations, with perfusion and oxygenation sensitive MRI. In cases where this is not available, we recommend careful interpretation of BOLD-FC results considering previous findings about hemodynamic-metabolic changes. In the future, accurate modeling of the hemodynamic-metabolic context might improve both our understanding of the crucial interplay between vascular-hemodynamic-neuronal components of intrinsic BOLD-FC and the evaluation of aberrant BOLD-FC in brain diseases with vascular-hemodynamic impairments.

Combining Gamma With Alpha and Beta Power Modulation for Enhanced Cortical Mapping in Patients With Focal Epilepsy.

About one third of patients with epilepsy have seizures refractory to the medical treatment. Electrical stimulation mapping (ESM) is the gold standard for the identification of "eloquent" areas prior to resection of epileptogenic tissue. However, it is time-consuming and may cause undesired side effects. Broadband gamma activity (55-200 Hz) recorded with extraoperative electrocorticography (ECoG) during cognitive tasks may be an alternative to ESM but until now has not proven of definitive clinical value. Considering their role in cognition, the alpha (8-12 Hz) and beta (15-25 Hz) bands could further improve the identification of eloquent cortex. We compared gamma, alpha and beta activity, and their combinations for the identification of eloquent cortical areas defined by ESM. Ten patients with intractable focal epilepsy (age: 35.9 ± 9.1 years, range: 22-48, 8 females, 9 right handed) participated in a delayed-match-to-sample task, where syllable sounds were compared to visually presented letters. We used a generalized linear model (GLM) approach to find the optimal weighting of each band for predicting ESM-defined categories and estimated the diagnostic ability by calculating the area under the receiver operating characteristic (ROC) curve. Gamma activity increased more in eloquent than in non-eloquent areas, whereas alpha and beta power decreased more in eloquent areas. Diagnostic ability of each band was close to 0.7 for all bands but depended on multiple factors including the time period of the cognitive task, the location of the electrodes and the patient's degree of attention to the stimulus. We show that diagnostic ability can be increased by 3-5% by combining gamma and alpha and by 7.5-11% when gamma and beta were combined. We then show how ECoG power modulation from cognitive testing can be used to map the probability of eloquence in individual patients and how this probability map can be used in clinical settings to optimize ESM planning. We conclude that the combination of gamma and beta power modulation during cognitive testing can contribute to the identification of eloquent areas prior to ESM in patients with refractory focal epilepsy.

Sensorimotor Representation of Speech Perception. Cross-Decoding of Place of Articulation Features during Selective Attention to Syllables in 7T fMRI.

Sensorimotor integration, the translation between acoustic signals and motoric programs, may constitute a crucial mechanism for speech. During speech perception, the acoustic-motoric translations include the recruitment of cortical areas for the representation of speech articulatory features, such as place of articulation. Selective attention can shape the processing and performance of speech perception tasks. Whether and where sensorimotor integration takes place during attentive speech perception remains to be explored. Here, we investigate articulatory feature representations of spoken consonant-vowel (CV) syllables during two distinct tasks. Fourteen healthy humans attended to either the vowel or the consonant within a syllable in separate delayed-match-to-sample tasks. Single-trial fMRI blood oxygenation level-dependent (BOLD) responses from perception periods were analyzed using multivariate pattern classification and a searchlight approach to reveal neural activation patterns sensitive to the processing of place of articulation (i.e., bilabial/labiodental vs. alveolar). To isolate place of articulation representation from acoustic covariation, we applied a cross-decoding (generalization) procedure across distinct features of manner of articulation (i.e., stop, fricative, and nasal). We found evidence for the representation of place of articulation across tasks and in both tasks separately: for attention to vowels, generalization maps included bilateral clusters of superior and posterior temporal, insular, and frontal regions; for attention to consonants, generalization maps encompassed clusters in temporoparietal, insular, and frontal regions within the right hemisphere only. Our results specify the cortical representation of place of articulation features generalized across manner of articulation during attentive syllable perception, thus supporting sensorimotor integration during attentive speech perception and demonstrating the value of generalization.